According to the Royal Mail press release:
Here's an example from Swindon Mail Centre dates 02-03-2017 (thanks MM):The postmark will appear on millions of items of UK stamped mail nationwide and will land on doorsteps on Saturday 4 March 2017. It will say ‘Imperial War Museums, 100 years’.Since 1917, IWM has collected - and displayed - a huge range of items that tell both civilian and military experiences and personal stories of war, and commemorate the sacrifices of all sections of society..Diane Lees, Director General of IWM said: “We are honoured that the Royal Mail has decided to honour IWM with a postmark to commemorate our centenary. IWM was established while the First World War was still being fought. Since then, people have entrusted us with their stories of war from 1917 to the present day, in the knowledge we will continue to share these stories with future generations”.The postal service provided a vital line of communication during the First and Second World Wars. During the First World War, the Army Postal Service handled a staggering two billion letters and 114 million parcels.David Gold, director of Public Affairs at Royal Mail, said: “Since the formation of the dedicated military postal unit, the Army Post Office Corps, in 1882, the postal service has played an integral role in times of conflict.“This close alliance remains as important now as it was then with this year not only marking 100 years since the creation of the War Museum but also the fourth issue in Royal Mail’s five-year series of stamps commemorating the First World War.
If the intention was that these would appear on mail delivered on 4 March then it is likely that the slogan was also in use on 3 March, especially for first class post.
UPDATE 21 MARCH
Thanks to JG for the National Apprenticeship Week slogan from South East Anglia MC which was used on 9 March (before Science Week slogan), but nobody noticed. Just shows how little stamped (and therefore postmarked) mail we get these days.
UPDATE 14 MARCH
It's British Science Week again marked by Royal Mail with the usual slogan postmark. Two different formats of course, for iLSM and IMP mail centres. Cornwall Mail Centre 13.03.17 and Warrington Mail Centre 10-03-2017
UPDATE 21 MARCH
Thanks to MC who sent a picture of his East Anglian Railway Museum PandG stamps on postcards. They arrived with this year's Mother's Day slogan from South East Anglia 20.3.17. They never show well on Post and Go of course.
If there are more March Slogans they will appear as we get news.